Halosilanes, such as trichlorosilane (TCS), are used in the manufacture of polycrystalline silicon. TCS can be prepared by a number of different processes, including hydrochlorination in which silicon tetrachloride (STC) is reacted with metallurgical grade silicon and hydrogen, often in a fluidized bed reactor, and the resulting crude TCS is subsequently purified prior to being fed into a CVD reactor in which the polysilicon is formed by deposition onto heated silicon filaments. The properties of the polysilicon produced depend, to a large extent, on the purity of the TCS used. In order to meet the stringent demands of the solar and semiconductor industries, it is critical to provide starting materials, including TCS, having extremely low levels of impurities.
Impurities in TCS can arise from a variety of different sources, including, for example, the metallurgical grade silicon from which it was prepared. This silicon is known to contain various metal species such as aluminum, iron, copper, phosphorus, and boron. Of these, boron-containing species have been found to be particularly difficult to remove from TCS. For example, boron-containing impurities typically have very similar boiling points as TCS, making separation of these contaminants extremely difficult and inefficient to accomplish by distillation. Furthermore, once the boron species have become entrained in the resulting polysilicon, it is also very difficult to remove. For example, boron-containing species partition nearly equally between a silicon melt phase and a solid phase, making it extremely difficult to remove by common re-solidification processes such as directional solidification. In addition, the presence of boron compounds provides an unwanted doping of the polysilicon properties, such that p-type semiconductors are obtained.
For this reason, various processes have been described for removal of boron-containing impurities from TCS. For example, it has been shown that boron species can be removed from TCS, either in the liquid phase or the gaseous phase, using an adsorbant such as silica gel. However, for such processes, the loading capacity of the adsorbant is often rapidly exceeded, and, as a result, an excessive quantity of adsorbant would be needed, making this approach uneconomical. In addition, it has been shown that water (such as from a moist inert gas) or other hydroxyl-containing species can be added to TCS to convert the boron-containing impurities, believed in the art to be primarily BCl3, into compounds that can be subsequently removed, such as by distillation. However, in such a process, an excess of hydroxyl-group reagent compared to the boron impurity is needed in order to fully remove the contaminants, and, under these conditions, TCS can also react, leading to unwanted side reactions and formation of silica and other polymeric siloxanes, which would require additional methods of removal.
Thus, while methods are known in the art, there is a need in the industry for improved methods and systems that are capable of efficiently and effectively removing contaminants, particularly boron-containing contaminants, from compositions containing trichlorosilane.